The MDL Programming Language : S.W. Galley and G. Pfister : Free Download, Borrow, and Streaming : Internet Archive with love from microfiche
The MDL programming language was one of the many variants of LISP developed in the 1970s, but some people may know it as the inspiration for ZIL that beget Z-machine that beget Zork and its ilk.
MDL was also called Muddle by its developers. It was developed on MIT’s PDP-10 machines running ITS in the early 70s, first as a joint project between the Dynamic Modeling and AI groups, but it quickly became an exclusively DM thing. On the DM PDP-10 it was extensively used for systems programming. Shortly before Zork there was another Muddle game popular on the ARPANET called Trivia. The DM group migrated off their aging PDP-10 to a new DEC-20 (also of the PDP-10 family) and ported their MDL environment over. Infocom brought Muddle along to their DEC-20 called Fred and used it to develop their games.
Current resurrection status is that TOPS-20 version of MDL has been found, and backported to ITS. A genuine ITS version has been located and used to compile and run Zork, but approval to release it is still pending.